Rejection of male skin grafts by females within an inbred strain of mice is caused by H-Y antigen, which cross-reacts serologically with a cell surface component found in the heterogametic sex of several vertebrate species including man. Thus serological assays for H-Y antigen offer a feasible and rigorous approach to the study of this cell surface component in humans with structurally modified and/or supernumerary Y-chromosomes. Using the sperm cytotoxicity test and the mixed hemadsorption-hybrid antibody test for H-Y antigen, we propose to study cells from patients with anomalous sex-chromosomal constitutions with a view to a) locating the gene which specifies or regulates H-Y antigen expression, b) evaluating the hypothesis that this locus functions in the primary determination of sex, and c) determining the efficacy of serological H-Y typing as a diagnostic tool in detecting Y-chromosomal genes in individuals lacking an apparent Y-chromosome and in sexing newborns with ambiguous external genitalia, with possible therapeutic implications.